Electrical cables for data transmission are well known. One common cable is a coaxial cable. Coaxial cables generally comprise an electrically conductive wire surrounded by an insulator. The wire and insulator are surrounded by a shield, and the wire, insulator and shield are surrounded by a jacket. Coaxial cables are widely used and best known for cable television signal transmission and ethernet standard communications in local area networks. Coaxial cables can transmit at much higher frequencies than a standard twisted pair wire and, therefore, have a much greater transmission capacity. Coaxial cables provide data transmission at raw data rates of up to 10 Mbps. In addition, coaxial cables have very little distortion, crosstalk or signal loss, and therefore, provide a very reliable medium for data transmission. Other types of cables are also well known, such as twisted pair cables used for telephone signal transmission, and fiber optic cables.
With the proliferation of high-speed, powerful personal computers and the availability of advanced telecommunications equipment, there is a need for cables which are capable of transmitting data at ever faster speeds. Fiber optic cables provide optimum data rate and performance for long distance and high data rate transmissions, since fiber optic cables provide very high data rate transmission with low attenuation and virtually no noise. Fiber optic cables provide data transmission at data rates up to and beyond 1 Gbps. However, despite the increased availability of fiber optic cables, the price of fiber optic cables and transceivers have not dropped to a level where it is always practicable to use. Accordingly, other less expensive cables capable of high speed data transmission are still in demand.
One such cable used for high speed data transmission between two points or devices is a parallel pair or twin axial cable. Parallel pair cable designs provide two separately insulated conductors arranged side by side in parallel relation, the pair being then wrapped in a shield. A common usage of these cables is to interconnect a mainframe computer to a memory device. As is well known, the speed and data rate with which the computer must communicate with the memory is critical to the computer's performance capabilities.
Parallel pair cables are often used for differential signal transmission. In differential signal transmission, two conductors are used for each data signal transmitted and the information conveyed is represented as the difference in voltage between the two conductors. The data is represented by polarity reversals on the wire pair, unlike a coaxial cable where data is represented by the polarity of the center conductor with respect to ground. Thus, the amplitude of the ground potential on a shielded pair cable is not significant as long as it is not so high as to cause electrical breakdown in the receiver circuitry. The receiver only needs to determine whether the relative voltage between the two conductors is that appropriate to a logical 0 or 1. Accordingly, differential signal transmission provides a better signal-to-noise ratio than voltage level to ground signal transmission (also called single-ended transmission) because the signal voltage level is effectively doubled by transmitting the signal simultaneously over the conductors, with one conductor transmitting the signal 180 degrees out of phase. Differential signal transmission provides a balanced signal which is relatively immune to noise and cross-talk. Interfering signals (or "noise") are generally voltages relative to ground and will affect both conductors equally. Since the receiver takes the difference between the two received voltages, the noise components added to the transmitted signal (on each wire) are negated. This noise is called common-mode noise, and the differential property of the receiver which negates the effect of this noise is known as common mode noise rejection. A standard for differential transmission systems is EIA standard RS-422.
As previously stated, parallel pair cable designs provide two separately insulated conductors arranged side by side in parallel relation, the pair being then wrapped in a shield. Most of the known parallel pair cable designs use a foil shield and include a third drain wire placed beside the parallel conductors. The two insulated conductors and the drain wire are then collectively shielded, often by being wrapped within a layer of aluminized polyester, and then the polyester layer is wrapped with an insulative and protective outer jacket layer, typically of polyvinylchloride (PVC).
In order to transmit the differential signal along a twin-axial cable effectively, the signals on each conductor must propagate down the wire with very low skew. The amount of differential skew per unit length that is allowable is inversely proportional to both the distance of the cable and the data rate at which signal is transmitted. For example, when transmitting at a data rate of 1000 Mbps, the bit width is approximately 1000 psec wide. If the difference between the two signals on the differential cable is greater than 200 psec, errors in communication may occur. If the differential signal is being transmitted 30 meters, then the safe maximum skew would be less than 7 psec/meter.
Unfortunately, for most existing twin-ax cables, typical differential skew is about 16-32 pSec/meter. This type of skew level limits the use-length of 1000 Mbps data transmission to less than 6 meters. As is discussed above, this significantly exceeds the safe level of skew for greater cable lengths. Accordingly, existing twin-axial cables are restricted in their ability to effectively transmit differential signals at a high data rate over an extended length.
Low differential skew is also required for proper cancellation of noise. If signals arrive at the receiver at different times, any coupled noise will not be able to cancel, defeating the primary purpose of a twin-ax cable. The present constraints on managing differential skew in conventional copper twin-ax cables severely limits the use of differential signal transmission in more demanding applications. Accordingly, many designers have been forced to switch to far more expensive fiber optic technology for long distance, high data rate transmission.
Therefore, it would be desirable to provide a cable capable of high data rate differential signal transmission at higher speeds and longer distances than achieved by existing twin axial cables. This requires having lower differential skew than is achieved by existing twin axial cables.